1. Field of the Invention
The present invention relates to a method of producing single crystals of silicon carbide or the like and a seed crystal used in the method.
2. Related Arts
Single-crystal silicon carbide (SiC) is useful for a substrate of a semiconductor device. As a method for producing the single-crystal SiC, the sublimation method in which source material powder is sublimated to be redeposited on a seed crystal is conventionally adopted, as reported in "Journal of Crystal Growth 43", 1978, PP. 209-212, for example. FIG. 1 schematically shows an apparatus used in the sublimation method. The apparatus includes a graphite crucible composed of a vessel 1 and a lid (a base) 2. SiC source material powder 3 is held in the vessel 1 so that a volume of the SiC source material powder 3 does not exceed a half volume of the vessel 1. On the lower face of the lid 2 is fixed a seed crystal 4 to oppose to the SiC source material powder 3. In a state that a temperature of the seed crystal 4 is kept to be lower than a temperature of the SiC source material powder 3 within the graphite crucible, the SiC source material powder 3 is sublimated. The gas sublimated from the SiC source material powder 3 is recrystallized on the seed crystal 4 to form single crystals 5.
In the above-mentioned method, the seed crystal 4 is usually bonded to the lid 2 through adhesives. In this case, however, it is difficult to bond the seed crystal 4 to the lid 2 without making any spaces therebetween due to low processing accuracy of the lid 2 and existence of the adhesives. Because of this, temperature of a surface of the seed crystal 4 bonded to the lid 2 (a bonding surface) becomes non-uniform depending on the area therein, thereby giving rise to recrystallization on the bonding surface of the seed crystal 4. The recrystallization starts from a lower temperature portion of the bonding surface, resulting in defects in the seed crystal 4. The seed crystal 4 further has a temperature gradient in a thickness direction thereof, and the temperature gradient functions as a driving power to develop the recrystallization in the seed crystal 4. Therefore, there is a possibility that the recrystallization reaches the other surface of the seed crystal 4 (a crystal growth surface) on which the single crystals 5 are formed.
This mechanism will be explained referring to FIGS. 2A to 2C. FIG. 2A shows a space formed between the seed crystal 4 and the base 21 due to the adhesives 6. In the process of growing the single crystals 5 by the sublimation method, the source material is kept at a higher temperature, while the seed crystal 4 is kept at a lower temperature. Therefore, a temperature gradient is generated in the entire portion in a crucible holding the seed crystal 4 and the source material. The temperature gradient also exists between the seed crystal 4 and the base 21 as well as inside the seed crystal 4.
Under this situation, when the seed crystal 4 is exposed to the space, sublimation and redeposition of SiC from the seed crystal 4 and surface diffusion of SiC on the surface of the seed crystal 4 occur so that SiC moves from a higher temperature portion to a lower temperature portion. For example, SiC moves from the seed crystal 4 to the base 21 and recrystallizes on the base 21 as shown in FIG. 2B. These phenomena accompany voids 41 in the seed crystal 4. In the seed crystal 4, SiC moves to occupy the voids 41 from the much higher temperature portion, thereby producing other voids 41 successively. Thus mass transfer of SiC progresses successively.
Consequently, the voids 41 exist between the recrystallized portion 42 and the other portion in which the mass transfer of SiC did not occur. The recrystallization occurs at several portions in the bonding surface of the seed crystal 4, and crystalinity of the recrystallized portion 42 is low. Further, the mass transfer of SiC is likely to occurs in defect part in the seed crystal 4, so that pipe-like defects (micro-pipe defects) 7 are generated to extend into the single crystals 5 formed on the seed crystal 4 as shown in FIG. 2C. As a result, crystalinity of the single crystals 5 is lowered, resulting in deterioration of yield factor of the single crystals 5.